Porous wet friction material utilizing a compliant epoxide resin binder system

ABSTRACT

A wet friction material is disclosed bonded to a plate for transmission of motion, with the friction material made up of a porous fiber substrate, which is at least partially impregnated with a catalyzed resorcinol diglycidyl ether resin to improve friction properties.

CROSS R FER NC

[0001] The present invention claims the priority of U.S. ProvisionalApplication No. 60/439,875, filed Jan. 14, 2003.

BACKGROUND OF THE INVENTION

[0002] The term “wet friction material” is well understood by thoseskilled in the art. These materials typically use a porous substratecomprising fibers such as wet laid cellulose and possible otheradditives. Rings of this substrate are typically mounted on each side ofa ring-shaped metal support disk. The substrate is partially impregnatedwith a curable resin dissolved in a solvent with the resin most widelyused being a phenol type. In a standard transmission or clutch, thesemembers are mounted in an interleaved relation with bare metal reactiondisks or plates, and the assembly can be moved in and out of engagement,for example, to provide a shifting or braking function in a vehicle.These assemblies run in lubricating liquids to provide, among otherthings, a cooling function.

[0003] In wet friction materials, it is desirable to have as high of acoefficient of friction as possible. Other desirable properties includedurability and high energy capacity.

[0004] It is taught in the literature that softer, generally referringto lower modulus, binder/substrate systems, have higher frictioncoefficients. The compliancy of the system, the ability to conform tothe reaction plate surface, maximizes surface contact during clutchengagement. This helps dissipate heat as well, reducing coning,hot-spotting, while improving durability.

[0005] Phenolic resin systems, in particular phenol formaldehyde—basedresoles, are the predominate saturating resins used in the frictionindustry. Unmodified, these resins are brittle and have relatively highmodulus. To lower modulus, improve fatigue resistance, and in generalimprove friction properties, these resin are modified, usually withelastomeric compounds such as nitrile or gum rubber. In addition, theymay be made more “linear”, hence more flexible, by using substitutedphenols, such as cresol, in place of phenol, and/or acetaldehyde,propionaldehye, benzaldehyde, etc., in place of formaldehyde. However,even extensive modification does not fully overcome the inherent brittlenature of phenolic systems. There are both chemical and practicallimitations, such as solution viscosity, which govern the possibleextent of chemical modification of these systems.

THE INVENTION

[0006] We have discovered that resorcinol diglycidyl ether (RDGE),catalyzed with a lewis acid or base to promote homopolymerization, whenused as a saturant or impregnating agent in a fiber substrate such aspaper, imparts unique and greatly improved friction properties. Frictioncoefficients are approximately 40-50% higher, and energy capacity 40-50% higher than phenolic systems.

[0007] The resin can be cured either with lewis acid catalysts, such asboron trifluoride or boron trichloride complexes, or with a tertiaryamine such as 2-ethyl, 4-methylimidazole (2,4-EMI) ortris-dimetlyaminomethyl phenol (DMP-30). It is important that thecatalyzation be homocatalytic to impart the desirable linear geometry tothe polymer. 2,4-EMI is the preferred catalyst.

[0008] In general, a high coefficient of friction is desirable becausemore torque can be transmitted to a transmission with a given frictionsurface area. This may allow design of transmission with fewer frictionplates, saving weight and space. While most paper substrates andtransmission fluids contain elements to modify friction coefficient, theRDGE binder provides additional performance benefits, as well as adynamic coefficient of friction of 0.17 and greater.

[0009] The resin is dissolved in a solvent, such as methyl ethyl ketone(MEK). It could also be dissolved in other ketones such as methylisobutyl ketone(MIBK), glycol ethers such as propylene glycol methylether (PGME), or mixtures of these with alcohols such as isopropylalcohol (IPA). The resin is diluted in solvent to a solids level of from35-50% depending on application. The curing agent is mixed into thesolvent/epoxy solution, where it has a pot life of from several days toseveral weeks depending on storage temperature.

[0010] After saturation, the parts are “B” staged for 10 minutes at 250F followed by 10 minutes at 400 F. The parts are then pressed to sizewith pressure and heat. If a post cure is required, the parts are postcured either prior to or after sizing. A typical post-cure would be 350F for one hour. The material is typically in disk or ring shaped form.

[0011] The friction material is bonded to a rigid or steel backingmember with adhesive. The friction material is essentially flat but maybe provided with outwardly facing grooves as well know in the art.

[0012] The RDGE resin can be combined with other resins without unduesacrifice to performance, for example epoxy novolac and others.Preferably, the RDGE amounts to at least 60% of the total resin, and theresin has a purity of greater than 80%. The RDGE resins are availablecommercially.

[0013] The RDGE resin, as well as it's various modifications with theRDGE resin include epoxy novolacs, bisphenol-A epoxy, aliphatic andaromatic epoxide reactive diluents such as cresyl glycidyl ether orcastor oil glycidyl ether, functionalized rubber compounds such ascarboxy, amine, or epoxide terminated acrylonitrile rubber, thermosetcompounds modified with these rubber compounds, and resole or novolacphenolic resins, all being thermoset resins, if necessary.

[0014] The paper base is a flat sheet of individual fibers, althoughother forms such as yarns, nonwovens, scrim, etc. could possibly beemployed. The typical base utilizes cellulose fibers, which is depositedas pulp on a porous surface and dried. Various additional fibers added,as well as additives, such as friction modifiers or other agents toimprove or alter performance. The paper sheet, of the desired uniformthickness, is dried using an oven or otherwise, and is then suitable forcutting into blanks prior to impregnation by the resin. These arestandard process and well known to those skilled in the art.

[0015] The term “wet friction material” refers to a relatively thinfiber base impregnated by resin and bonded to a rigid or steel backingplate. These plates are arranged in alternating series with a relativelyrotating steel reaction disk or plate to transfer power in the assemblyrunning in oil or liquid lubricant, by nature of friction. Due to theextreme conditions of activation, engagement and disengagement, thefriction material must possess, for example, good wear properties,thermal stability, compliance with the reaction plate, and the desireddegree of friction coefficient without detriment to other essentialproperties.

EXAMPLE

[0016] An automatic transmission friction assembly was prepared.Raybestos 7049 production paper 0.023″ thick was cut to size and bondedto both sides of a steel circular disk, under heat and pressure, with aphenolic adhesive. The assembly was then placed into a resin solutionand the paper saturated, so that a resin pick-up of 55% (+or −5%) of thedry weight of the paper as described below was achieved. The assemblieswere then run through a drying oven to remove solvent and partiallycure, or “B” stage the resin. The parts were then pressed to final size,at 500 F under pressure, to 0.017″ bonded paper thickness on each side,followed by an oven post-cure which was optimal for the resin system.

[0017] Three of these composite assemblies were then placed in a testassembly and tested in the SAE J2487 Test Procedure as described above.

DEFINITIONS

[0018] 1. 7049 can be described as a wet—layed cellulose—base papermodified with diatomaceous earth and a latex binder. MidpointCoefficient—The coefficient of friction taken from the midpoint of theTorque Curve.

[0019] 2. SAE J2487 —A stepped power level test. In this procedure,three doubled-sided friction assemblies are assembled in a clutch packwith steel reaction plates contacting each friction surface (fourreaction plates). A transmission oil is specified. The assembly isaccelerated to 3600 rpm, then stopped in 0.5 seconds by interaction ofthe friction surfaces with the reaction plates. At each subsequent testlevel, the inertia of the engagement is increased, thus requiring moreforce to stop the assembly in 0.5 seconds. Failure is determined when 1)the friction material fails, i.e., delaminating or tearing, or 2) whenheat transfer results in coning (warping) of the reaction plates.

[0020] 3. Epoxy Novolac—An epoxidized phenol-formaldehyde novolac resin,with an epoxide equivalent weight of 170, and viscosity of 30,000centipoise @ 25 C.

[0021] 4. CTBN Modified Bisphenol-F Epoxy Resin—An epoxidizedphenol-formaldehyde novolac resin, with an epoxide equivalent weight of165, and viscosity of 3200 cps @ 25 C, modified 20% with CTBN rubber.

[0022] 5. 2,4-EMI—2-ethyl, 4-methylimidizole

[0023] 6. 7049-H4—7049 paper saturated with a resin solution of: 80%RDGE, 10% epoxy novolac resin, and 10% CTBN modified Bisphenol-F epoxyresin, diluted to 35% solids in methyl Ethyl Ketone (MEK). Catalyst2,4-EMI is added at 3% based on resin (BOR) weight.

[0024] 7. 7049-H3—7049 paper saturated with a resin solution of: 70%RDGE, 20% epoxy novolac, and 10% CTBN modified Bisphenol F epoxy resin,diluted to 35% solids in MEK. 2,4-EMI is added at 3% BOR.

[0025] 8. 7049-H2—7049 paper saturated with a resin solution of: 70%RDGE and 30% epoxy novolac resin, diluted to 35% solids in MEK. 2,4-EMIis added at 3% BOR.

[0026] 9. 7049-H1—7049 paper saturated with a resin solution of: RDGEdiluted to 35% solids in MEK. 2,4-EMI is added at 3% BOR.

[0027] 10. 7049-P—7049 paper saturated with a production resol phenolicresin, diluted to 35% solids in ethanol. No catalyst is added.

[0028] 11. Hmp—Midpoint Coefficient of Friction

[0029] b 12. Hep/Hmp—Endpoint Coefficient of Friction divided byMidpoint Coefficient of Friction (E/M) ratio.

[0030] 13. Hbk—Breakaway Coefficient

[0031] b 14. mm—Wear in millimeters

[0032] 7049-H1 was assembled, post cured 1 hour at 350 F, and evaluatedas described above.

Example 2

[0033] 7049-H2 was assembled, post cured 1 hour at 350 F, and evaluatedas described above.

Example 3

[0034] 7049-H3 was assembled, post cured 1 hour at 350 F, and evaluatedas described above.

Example 4

[0035] 7049-H4 was assembled, post cured 1 hour at 350 F, and evaluatedas described above.

Comparative Example 5

[0036]7049-P was assembled, and evaluated as described above. No postcure was needed.

Results

[0037] The table below summarizes and compares performance of theexample formulations at Test Level 5. Notice that the frictioncoefficient of 7049-H1 at that level is 52% higher than 7049-P. 7049-H1ran to 11 levels vs 7 levels for 7049-P, demonstrating higher energycapacity. 7049-H1 also displays a lower E/M ratio. All of the epoxyformulations show significantly higher friction coefficients and energycapacity. TABLE 1 Performance Comparison LEVEL 5 @ CYCLE 200 LevelsResin Hmp Hep/Hmp Hbk mm Completed 7049-H1 0.169 1.006 0.117 0.0965 117049-H2 0.166 1.054 0.119 0.0745 9 7049-H3 0.157 1.045 0.118 0.0948 107049-H4 0.173 1.012 0.119 0.0965 10 7049-P 0.111 1.126 0.127 0.1465 7

[0038] Table 2 summarizes Storage Modulus and Glass TransitionTemperature (Tg), determined by Dynamic Mechanical Analysis (DMA). Thelower modulus of the epoxy systems translates into better conformity atoperating temperature. TABLE 2 Comparison of S Modulus and Tg Modulus(Mpa) Material @ 150 C. Tg ° C. 7049-H1 500 130 7049-H2 750 145 7049-H3450 135 7049-H4 400 130 7049-P 1600 >250

What is claim d Is:
 1. A wet friction material comprising a basecontaining fibers, and a resin at least partially impregnating saidbase, said resin comprising at least 60 percent of a catalyzedresorcinol diglyoidyl ether.
 2. The wet friction material of claim 1,wherein the resin is diluted in a solvent prior to application to thebase, and the solvent is evaporated.
 3. The wet friction material ofclaim 1, wherein the material is essentially thermosetting.
 4. Thefriction material of claim 1, wherein the base comprises cellulosefibers.
 5. The friction material of claim 1, wherein the frictionmaterial is essentially a flat disk, and a rigid flat plate adhered tothe flat disk.